The present invention relates to an improvement in a switching power supply, and more particularly to a high-frequency switching power supply suited to drive the magnetron for microwave ovens.
There has been well known a switching power supply provided with a step-up transformer, in which an input voltage is applied across a primary winding, and a high voltage developed across the secondary winding is supplied to a load, for example, a magnetron. In this power supply, a switching element such as a transistor is connected in series with the primary winding, and a resonating capacitor is connected in parallel to one of the primary winding, the secondary winding and the switching element. When the switching element repeats an ON-OFF operation at high speed, high-frequency oscillation is generated in, for example, a resonance circuit made up of the capacitor and the primary winding, and a high voltage is developed across the secondary winding.
An example of a switching power supply is disclosed in Sanyo Technical Review, Vol. 14, No. 1, Feb., 1982. In this switching power supply, the switching element is connected in series with the primary winding, and the resonating capacitor is connected in parallel to the switching element.
In such a switching power supply, a rectified voltage is applied to the primary winding of the step-up transformer in the ON-period of the switching element, and the energy stored in the primary winding in the ON-period is supplied to the resonance circuit made up of the capacitor and the primary winding, in the OFF-period of the switching element. Thus, a current flows through the primary winding in the OFF-period. Further, the output of a magnetron applied with a high voltage from the secondary winding is adjusted by changing the ON-period of the switching element, that is, the duty ratio of the anode current of the magnetron. In this case, the voltage applied across the switching element is required to become equal to zero in the OFF-period of the switching element. Hence, it is desirable to make small the inductance of the resonance circuit. If a large residual voltage exists, a large current due to the residual voltage will flow through the switching element, and there will be a fear of damaging the switching element.
The voltage waveform, which is applied across the switching element in the OFF-period thereof, is determined by the electric constants of the primary winding of the step-up transformer, the resonating capacitor connected in parallel to the primary winding or the switching element, and a high-tension capacitor provided on the secondary side. In general, the inductance value of the primary circuit of the step-up transformer is appropriately set to obtain a favorable oscillating waveform. This is because sufficient magnetic energy has to be stored in the ON-period of the switching element by the inductance of the step-up transformer, and the inductance of the step-up transformer is required to be greater than a predetermined value.
Further, unless the peak value of the anode current of the magnetron is made less than a predetermined value, for example, 1.2 A, moding may occur so that an excessively high voltage is generated in the magnetron. In order to prevent the generation of such an unordinary voltage, the primary or secondary circuit of the step-up transformer is required to have the inductance of an appropriate magnitude. Thus, it is necessary to determine the inductance values of the primary and secondary circuits in accordance with the above objects and independently of each other.
In general, the primary inductance L.sub.01 of a transformer at a time the secondary side is made open, that is, the inductance L.sub.01 viewed from the primary side is nearly equal to the reduced or converted secondary inductance L.sub.02 which is obtained by converting the secondary inductance L.sub.2 of the transformer at a time the primary side is made open (that is, the inductance viewed from the secondary side) into an inductance value on the primary side. That is, L.sub.01 = hd 02 = a.sup.2 L.sub.2 where a indicates a transformation ratio (namely, turn ratio). In view of the above fact, it has been proposed to insert a reactor in one of the primary and secondary circuits for the purpose of adjusting the inductance values of these circuits independently of each other. In this case, however, a switching power supply becomes, as a whole, large in size, and the manufacturing cost thereof is increased.